Somatomedin synthesis by a subclone of Buffalo rat liver cells. Influence of divalent cations on rIGF-II release

Previous studies have provided evidence that biosynthesis and secretion of somatomedin (SM) is not only hormone dependent, but also modulated by nutritional factors. Little is known, however, about the role of divalent cations in these processes. A subclone of Buffalo rat liver (BRL) cells, known to secrete rat insulin-like growth factor-II (rIGF-II) into serum-free medium, was used to define the influence of Ca2+, Mg2+ and Zn2+ on this function. The secretion of rIGF-II by subclone BRL-3SC appears to be quite stable in minimal essential medium (MEM) over a wide range in Ca2+ concentrations (0.18–3.0 mm) but is reduced to only 8% of controls in the absence of Ca2+ (P <0.01). Reducing, and even eliminating, the extracellular concentration of Mg2+ alone caused no change in basal rIGF-II release, while simultaneously decreasing Mg2+ and Ca2+ results in a marked drop in the secretion of this SM, reaching a nadir of 38% of controls in the absence of Mg2+ (P <0.001). A Mg2+ concentration of 10 mm, or 25 times 'normal', did not alter the basal secretion of rIGF-II. Eliminating the trace amount (0.8 nm) of Zn2+ in MEM by chelation with EDTA decreased rIGF-II secretion to 62% of control levels (P<0.01), while increasing the concentration of this cation to 3 mm did not alter the basal release of this SM. Decreased rIGF-II release in the presence of EGTA and EDTA is not due to irreversible cell damage since the secretion of this SM was restored to normal during subsequent reincubation in MEM alone. These studies indicate that normal rIGF-II secretion by BRL-3SC cells occurs only at concentrations of Ca2+, Mg2+ (in the presence of reduced Ca2+) and Zn2+ above certain threshold levels. Moreover, reduced Mg2+ alone, without a concomitant reduction in Ca2+, does not decrease rIGF-II release, attesting to the known interaction of these cations in a variety of intracellular processes. Concentrations of these three cations considerably higher than 'normal' appear to have no significant effect on basal rIGF-II secretion.